Cooley, Thomas M.
Little, Brown, 1898
Buku Teks SO
Fincher, E. B.
Abelard-Schuman, 1955
Buku Teks
New York United Nations 1989,
Buku Teks
Risjord, Norman K.
Prentice-Hall, 1985
Buku Teks SO
Stuart, Graham Henry, 1887-
Appleton-Century-Crofts, 1955
Buku Teks SO